1. Field of the Invention
The present invention relates to an unsaturated polyester resin composition, to a molding produced with the use of this composition, and to a production process of the molding, which unsaturated polyester resin composition can provide laminate moldings or joint monolithic moldings of an unsaturated polyester resin satisfactory in adhesiveness (secondary adhesiveness) between cured articles of unsaturated polyester resins, especially of glass fiber reinforced unsaturated polyester resins.
2. Description of the Related Art
Fiber reinforced plastic products using unsaturated polyester resins are light in weight, satisfactory in corrosion resistance, heat resistance and mechanical strength and high in moldability, and are in wide use as materials for plastic moldings, in especial as matrix resins for fiber reinforced plastic moldings, which require minimized weight and high heat resistance, corrosion resistance and strength, or as back coat layers or gel coat layers of moldings composed of a gel coat layer and a back coat layer.
In this connection, unsaturated polyesters each having a dicyclopentenyl group have been known as resins which are good in compatibility with styrene and low in volatility of styrene, which styrene is contained in the resins as a vinyl monomer for crosslinking (hereinafter may simply referred to as a "crosslinking vinyl monomer"). In an air-contact surface of these unsaturated polyesters, namely a non-contact surface with a mold of these unsaturated polyesters applied in an open mold, curing proceeds fast because of air curing due to the dicyclopentenyl group, and the unsaturated polyesters are advantageously used as molding materials to be used in an open mold.
Cured articles of glass fiber reinforced unsaturated polyester resins are generally obtained by impregnating a glass fiber, a reinforcement, with an unsaturated polyester resin to form a laminate, and curing the resin. The cured articles are, however, hardly formed by a single step when they are wide in lamination area and large in thickness, e.g., 8 mm or more. In this case, the cured articles are generally obtained by, for example, a process comprising the steps of forming a first cured article about 3 to 5 mm thick beforehand, and forming a second cured article on the surface of the first cured article by wet lamination to give a molding composed of joined first and second cured articles, and repeating this procedure, or a process comprising the steps of molding a plurality of first cured articles beforehand, and joining these first cured articles with the use of a glass fiber reinforced unsaturated polyester resin, and curing the resin.
However, when an unsaturated polyester resin having a dicyclopentenyl group is used as a resinous molding material, and a second cured article is to be formed on the surface of a first cured article after the lapse of a certain time from the formation of the first cured article, the surface of the first cured article has been cured completely, and a laminated interface between the first and second cured articles becomes low in adhesiveness, which invites interfacial peeling of the laminate.
A possible solution to this peeling problem is to ensure secondary adhesiveness in laminate moldings or joint monolithic moldings using such unsaturated polyester resins each having a dicyclopentenyl group. For this purpose is employed a technique of forming a first cured article, and roughing the surface of a secondary adhesive surface of the first cured article by, for example, sanding to ensure secondary bond strength through anchoring effect.
Such additional processing as surface sanding, however, increases the number of process steps, which inhibits improvement in productivity. In addition, when the first cured article is a molding of a glass fiber reinforced plastic, the glass fiber is atomized in the sanding process and rises into air to deteriorate working atmosphere.
Accordingly, there is a strong felt need to provide unsaturated polyester resins which can ensure secondary adhesiveness without, for instance, sanding of the surface of a first cured article.
Separately, the use of methyl methacrylate or another polymerizable aliphatic vinyl monomer as part of crosslinking vinyl monomers is well known to improve the weather resistance of resultant moldings. Methyl methacrylate is, however, highly anaerobic (i.e., low in affinity to air) and its air-contact surface is cured very slowly. Accordingly, if a molding having a gel coat layer containing methyl methacrylate is obtained by forming the gel coat layer in an open mold, laminating a resinous molding composition on an air-contact surface of the gel coat layer, and curing the resin (hereinafter a layer formed by lamination may be referred to as "back coat layer"), it shows lifting and thus becomes defective. To be more specific, when an unsaturated polyester resin composition containing a polymerizable aliphatic vinyl monomer as part of crosslinking vinyl monomers is applied and cured to form a gel coat layer, and a resin composition for back coating is laminated onto the gel coat layer, a polymerizable vinyl monomer in the back coating resin composition migrates into the gel coat layer, which has been insufficiently cured, to swell the gel coat layer. The swelling of the gel coat layer invites creasing, crimps and uneven lifting of the gel coat layer itself and thus causes a defect, so-called lifting. A possible solution to this problem is to laminate a back coating resin composition after the gel coat layer is cured sufficiently. In this case, however, the productivity is deteriorated, and the adhesiveness between the gel coat layer and back coat layer is decreased due to the aforementioned deteriorated secondary adhesiveness.
The unsaturated polyester resins each having a dicyclopentenyl group are, as compared with unsaturated polyester resins having no dicyclopentenyl group, liable to exhibit yellowing due to long-term exposure to outdoor weathering and to be low in surface gloss. When they are used as moldings for outdoor applications, their weather resistance should essentially be improved. In particular, as unsaturated polyester resins obtained predominantly from styrene or another polymerizable aromatic vinyl monomer become further low in weather resistance, and demands for improvement in weather resistance have increased. In this connection, such polymerizable aromatic vinyl monomers are compounded for the purpose of ensuring the strength and heat resistance required for moldings.